The present invention relates to rosin emulsions for use in the sizing of paper, board and like materials and also relates to a method of sizing using the rosin emulsion.
It is well established practice to effect paper sizing by use of a rosin emulsion and a soluble aluminium salt such as aluminium sulphate (e.g. in the form of paper-makers alum [Al2(SO4)3.16-18H2O], aluminium chloride, poly-aluminium chloride or aluminium chlorohydrate. (Although the term xe2x80x9cemulsionxe2x80x9d is conventionally used to describe the rosin product it should more properly be called a dispersion since although made as an emulsion, the particles are effectively solid at ambient temperature). The rosin emulsion and aluminium salt are generally used as separate additions to the paper making process although it is also known to formulate so-called xe2x80x9cone-shotxe2x80x9d compositions which are formed by pre-mixing a rosin emulsion and an aluminium salt.
One method of producing a rosin emulsion is by the well established Bewoid process as disclosed in U.K. Patent No. 335 902. In this process, sizes with about 90% free rosin are produced using a protective colloid (casein) as the stabilizer and a small amount of rosin soap as the dispersant. In this process, approx. 1-2% of sodium hydroxide or potassium hydroxide on the weight of rosin is used to form a rosin soap dispersant by adding it to the molten rosin (usually at a temperature of about 130xc2x0 C.) so as partially to saponify the rosin, followed by the casein dissolved in approx. 10% of its weight of sodium hydroxide (although potassium hydroxide or borax are occasionally used instead). Finally, water at ambient temperature is added to produce a dispersion at a much lower temperature, usually containing between 30 and 50% solids. During the cooling back process, emulsion inversion occurs in that the initially formed water-in-rosin emulsion inverts as more water is added to give a rosin-in-water emulsion. As the temperature of the emulsion falls, the rosin emulsion droplets solidify to give a dispersion of rosin in an aqueous medium (i.e, the so-called rosin emulsion).
The rosin emulsion so formed is anionic by virtue of the presence of the anionic dispersant (rosin soap) and the casein stabilizer. Such anionic dispersions are conventional and are extensively used for sizing paper, board and like materials.
There are also disclosures in which casein has been replaced by anionic surfactants. Thus, U.S. Pat. No. 4,199,369 describes the use of a specific dispersant that is of the type alkylethyleneoxide sulphites without the use of an additional stabiliser in an inversion process leading to the production of an anionic emulsion.
Cationic sizing emulsions are also known and are preferred for various applications in that they arc considered to provide improved sizing efficiency as compared to anionic dispersions. Generally, commercial cationic rosin emulsions are produced by homogenization of rosin at high temperature and high pressure (usually approx. 160xc2x0 C. and 15 bar respectively) using a dispersant and a polymeric cationic stabilizer. Instead of lowering the viscosity by raising the temperature to greater than 160xc2x0 C., an alternative is to dissolve the rosin in a solvent (e.g. dichloromethanexe2x80x94see EP 0 719 892 and EP 0 719 893) to give a solution of low viscosity which can then be homogenized at a temperature below 100xc2x0 C. The stabilizer is usually a synthetic cationic polymer or it can be a cationic starch. In this respect, it should be noted that attempts to prepare cationic rosin emulsions by a xe2x80x9cBewoid-typexe2x80x9d inversion process but using a cationic stabilizer instead of the anionic casein have generally been unsuccessful although U.S. Pat. No. 4,983,257 discloses an inversion process for producing a rosin emulsion by an inversion process in which pan of the casein is replaced by a cationic starch.
A development relating to cationic emulsions is disclosed in WO-A-9824972 (Roe Lee Paper Chemicals Co. Ltd.) in which a xe2x80x9cone-shotxe2x80x9d sizing composition is produced from an admixture of an anionic rosin emulsion, a soluble aluminium salt and a cationic starch derivative which has been produced by a process in which the starch structure was split prior to the cationizing step (e.g. as available under the trade mark RAIFIX). The requirement for the initial production of an anionic emulsion is clearly a disadvantage in that it increases the number of steps required to produce the cationic emulsion. There is also the disadvantage of the need to store the anionic emulsion before conversion to the cationic product. Because casein and a cationic stabilizer are used, the total raw material cost can be greater than producing a cationic rosin emulsion by direct homogenization of rosin, a dispersant and a cationic stabilizer.
It is therefore an object of the present invention to obviate or mitigate the abovementioned disadvantages.
According to a first aspect of the present invention there is provided a cationic rosin-in-water emulsion which has been prepared without intermediate isolation of an anionic rosin-in-water emulsion and in which the dispersed rosin phase is stabilised by a cationic polymer derived from a degraded starch, said polymer having a degree of substitution of at least 0.15 quatenary groups per glucose unit.
According to a second aspect of the present invention there is provided a method of producing a cationic rosin-in-water emulsion comprising effecting emulsification of rosin in water in the presence of a cationic polymer derived from a degraded starch, said polymer having a degree of substitution of at least 0.15 quatemary groups per glucose unit.
The invention has been based on our discovery that cationic polymers derived from degraded starch (as defined in the previous paragraph) may be used for directly producing cationic rosin size emulsions from rosin without the need for preliminary isolation of an anionic rosin emulsion, avoiding the disadvantages of the prior art as discussed above.
The rosin emulsion of the invention may be produced in various ways which are described more fully below. Briefly, however, the emulsions may be prepared by an inversion process in which an emulsion of water in molten rosin is xe2x80x9cinvertedxe2x80x9d in the presence of the cationic polymer derived from degraded starch to produce a rosin-in-water emulsion. A further possibility is for the rosin emulsion to be produced by homogenization of a rosin and the cationic polymer.
The important feature of the invention is the use of a cationic polymer derived from a degraded starch and having a degree of substitution of at least 0.15 quaternary nitrogen groups per glucose unit. Such polymers may be produced, from starch, by splitting the starch structure and then effecting cationization producing a polymer which has a very high level of cationicity. Such starch derivatives may be produced with a range of relative molar masses and degree of cationization.
Preferably the cationic polymer used in the present invention has 0.15 to 1.30 quaternary groups per glucose unit. More preferably, this degree of substitution is from 0.20 to 1.10, even more preferably from 0.20 to 0.80 and most preferably from 0.50 to 0.80 quaternary groups per glucose unit.
The degree of substitution (i.e. at least 0.15 quaternary groups per glucose unit) in the cationic polymers employed in the present invention is considerably higher than that found in the cationic starches conventionally used for producing cationic emulsions for use in paper sizing. Thus, the cationic polymers have a higher charge density than the conventional starches. For example, conventional cationic starches are generally considered to have a charge density expressed in milli-equivalents per gramme (i.e. the average number of milli-equivalents of quaternised nitrogen per gramme of the polymer) in the range of 0.1-0.3. Typically the cationic polymers employed in the present invention have a charged density of 1.0 to 3.5 milli-equivalents per gramme. Whilst we do not wish to be bound by theory, we believe that the success of the present invention is due to this higher charge density and lower relative molar mass.
The starch derivatives used in the present invention may be produced by treating starch with an agent which will partially xe2x80x9cfragmentxe2x80x9d the polysaccharide chain (e.g. by use of an oxidising agent such as hydrogen peroxide) and subsequently quaternising the xe2x80x9cfragmentedxe2x80x9d product in a reaction in which the solids content is a minimum of 50% (more preferably at least 55%) by weight. The cationizing agent may be used in an amount of 20 to 160%, most preferably 80 to 120%, by weight of the amount of starch thereby allowing the degree of cationization to be readily controlled.
The preferred cationizing chemical is 2,3-epoxypropyl trimethyl ammonium chloride although other cationizing agents may be used.
Preferred starch derivatives for use in the invention have a nitrogen content in the range of approximately 1-5% (for the cationizing chemical being 2,3-epoxypropylene trimethyl ammonium chloride). It is particularly preferred that cationic, oxidised starch derivatives for use in the invention have a nitrogen content of 3-5%, more preferably 3-4%, and ideally about 3.5% (for the cationizing chemical being 2,3-epoxypropylene trimethyl ammonium chloride).
The relationship between the nitrogen content of the cationic polymer (quatemised using 2,3-epoxypropyl trimethyl ammonium chloride) and degree of substitution per glucose unit is illustrated in FIG. 1 of the accompanying drawings. It can be seen from the graph of FIG. 1 that the polymers mentioned above containing 3-4% of nitrogen have a degree of substitution in the range of about 0.53 to about 0.8.
It is preferred that the starch derivative for use in the invention has a molecular weight in the range of 100,000 to 500,000 Daltons, more preferably 150,000 to 450,000.
Further details as to the manner in which the starch derivatives may be manufactured are given in WO-A-95/18157 and Finnish Patents Nos. 94135 and 94128 (all in the name of Raisio Chemicals). The disclosures of these prior specifications are hereby incorporated by reference.
Particularly suitable quaternised, oxidised starch derivatives for use in formulating the emulsions of the invention are available under the trade mark RAIFIX which designates a range of products of varying cationicity and molecular weight. RAIFIX products are available under the following five figure product codes:
In these product codes, the first three figures are representative of molecular weight (the higher the number the higher the percentage molecular weight) and the final two figures being approximately ten times the nitrogen content. Estimates of the relative molar mass of the RAIFIX products are 100,000 Daltons for the 010xx designations and 500,000 Daltons for the 250xx designations. Preferred RAIFIX products for use in the invention have a molecular weight designation of at least 070 and a nitrogen designation of at least 35. Particularly suitable RAIFIX products for use in the invention are available under the designation 07035 and 15035.
The rosin used in the emulsion of the invention may be for example be a wood rosin, gum rosin, tall oil rosin or mixtures thereof. The rosin may be maleated, fortified, unfortified, hydrogenated or disproportionated. The rosin may optionally be esterified. If a tall oil rosin is used then it is preferred that it be pre-treated with paraformaldehyde.
The preferred rosin for use in the invention is a maleated or fumarated rosin.
If desired, the rosin may be compounded with tall oil fatty acids as proposed in UK-A-2 268 941. Tall oil fatty acids are available for example under the designation CENTURY MO6 (ex Union Camp Chemicals Limited). Any other rosin diluent (e.g. a hydrocarbon resin) may alternatively be used.
Rosin emulsions in accordance with the invention may, for example, typically have a solids content of 60% by weight or less, e.g. 55% by weight or less. In more preferred embodiments of the invention, the rosin emulsion will have a maximum solids content of 50% by weight, more preferably a maximum of 40% by weight and even more preferably a maximum of 30% by weight. The rosin (together with all tall oil fatty acids present) may comprise 75% to 95% by weight of the total solids of the rosin emulsion and the cationic starch derivative may correspondingly comprise 5% to 25% of the total solids. In a particularly suitable rosin emulsion in accordance with the invention, the rosin (including any tall oil fatty acids) provide about 87 to 93% by weight of the total solids.
As indicated briefly above, a rosin emulsion in accordance with the invention may be produced by an inversion process in which, in a manner akin to the Bewoid process (but using the cationic starch derivative as stabilizer instead of casein) an emulsion of water in molten rosin is inverted to produce a rosin-in-water emulsion.
A particularly preferred method of producing an emulsion in accordance with the invention by an inversion process comprises the steps of
1. Melting the rosin and stirring the molten material (e.g. in a standard Bewoid pot).
2. Either saponifying some of the rosin or treating the rosin with a dispersant, e.g, naphthalene sulphonic acidxe2x80x94formaldehyde condensate or sodium ligno-sulphonate, or a dispersant as disclosed in U.S. Pat. No. 4,199,369.
3. Cool back the rosin with water, heated somewhat if necessary, until the temperature is just below 100xc2x0 C.
4. Adding a solution of the cationic starch derivative, preferably at an elevated temperature, to the stirred molten rosin.
5. Continuing to stir the mixture of rosin and cationic starch derivative for at least several minutes which serves to mix thoroughly the starch solution with the rosin, and the experiments we have performed has helped in the control of the particle size of the final emulsion.
6. Adding dilution water (sometimes it is of assistance to use heated water at 80-95xc2x0 C. whilst stirring until approximately 50% solids is attained. During this process the emulsion inverts to a rosin-in-water emulsion.
7. Adding further quantities of water to produce an emulsion of the required solids content.
In step 4 of the above process, the solution of the cationic starch derivative is preferably heated to a temperature of at least 85xc2x0 C. and more preferably to a temperature close to that of the cooled back rosin prior to addition to the molten rosin. In step 5, the water is preferably but not necessarily heated to a temperature of at least 80xc2x0 C. more preferably at least 90xc2x0 C. before addition to the molten rosin. Without use of the elevated temperature additions in step 4 and 5, it may be found that the these additions can quickly cause undesirable thickening (and possibly solidification) of the rosin.
Furthermore, it may be found that (at least for certain rosins) the viscosity of the emulsion at all stages is higher than during preparation of standard anionic emulsions. This can cause inefficient mixing and beating resulting in a poor particle size. The problem may however be readily overcome by substituting up to 20% by weight, ideally about 5 to 10% by weight, of the rosin with tall oil fatty acids. (e.g. as available under the designation CENTURY MO6). Alternatively or additionally, the speed of mixing may be increased as compared to production of a conventional anionic rosin emulsion.
An alternative method of producing the emulsion is in add molten rosin, with vigorous stirring to heated water (e.g. 85-95xc2x0 C.) containing a saponifying, agent (e.g. NaOH) or a dispersant for the rosin. Subsequently the starch derivative is added followed by dilution water. Provided that the volume ratio of rosin to total aqueous phase (prior to addition of dilution water) is greater than about 60:40 and that by addition of dilution water this ratio is reduced to below 40:60 then such a production method usually involves conversion of a water-in-rosin emulsion to a rosin-in-water emulsion.
A further alternative method of producing emulsions in accordance with the invention is by homogenization of molten rosin and a solution of the cationic starch derivative using conventional homogenization equipment. If it is desired to effect homogenization at a temperature of less than 100xc2x0 C. then it is preferred that up to 20% by weight, ideally about 5 to 10% by weight, of the rosin is substituted by tall oil fatty acids. A more commercial method would be to omit the tall oil fatty acids from the formulation, and opt for higher temperatures where the viscosity of the rosin is low enough to permit direct homogenisation of the rosin. This approach would suggest temperatures of the order of 160xc2x0 C. and pressures greater than the vapour pressure of water at this temperature.
An alternative to effecting homogenization of molten rosin is for the rosin to be dissolved in a suitable solvent, e.g. dichloromethane, and the solution homogenised with water and the cationic starch derivative prior to solvent removal.
Rosin emulsions in accordance with the invention may be used for the internal or surface sizing of paper, paper board and like materials using well established techniques. Thus, for example, sizing may be effected by use of the rosin emulsion and a soluble aluminium salt such as aluminium sulphate, e.g. in the form of paper-makers alum [Al2(SO4)3.16-18H2O], aluminium chloride, poly-aluminium chloride or aluminium chlorohydrate. The amount of rosin emulsion used will typically be such that the amount of rosin is 0.2-3% by weight dry basis on the weight of fibres.
The rosin emulsion and aluminium salt may be added as separate additions to the wet end of the paper making process. The rosin emulsion and aluminium salt may, for example, be added using the techniques disclosed in WO-A-91/02119. It is however also possible to formulate a xe2x80x9coneshotxe2x80x9d composition, i.e. a formulation in which the rosin emulsion and the aluminium salt are mixed together before being supplied in a single container. xe2x80x9cOne-shotxe2x80x9d products made from conventional anionic Bewoid emulsions require that agitation is continued beyond formulation of an initial precipitate to convert the latter into an emulsion (see for example GB-A-2 050 453) which discloses production of xe2x80x9cone-shotxe2x80x9d emulsions from an anionic Bewoid rosin emulsion and an aluminium salt.